One technique currently applied in tachometer systems to measure speed, counts, for a fixed period of time, pulses generated as a function of tachometer shaft motion. At low speeds when shaft rotation is slow, the number of pulses generated is low and the accuracy of the system is poor. One approach to improve the number of pulses produced and the accuracy at low speeds is to use step-up gear train between the sensing wheel and shaft to increase the speed of the shaft. This increases the cost and inertia of the system, the latter of which may detract from the use of the system to measure the speed of delicate members. Generally the accuracy of such devices is improved by increasing the number of pulses generated per shaft revolution. However, higher pulse rates require more complex equipment. For example, in those devices using shutter wheels to produce pulses, any increase in the number of shutter wheel apertures increases the cost of the wheel and the associated optics. Further, with a large number of apertures any, even minor, disturbance or vibration may cause the generation of two pulses where there should have been only one.
In another technique, clock pulses occurring between pulses generated by shaft rotation are counted to determine the period of the shaft pulses. The period may then be inverted to determine the rate. This technique may result in poorer accuracy at higher speeds.